This invention relates to a diode structure having the parts thereof selected and dimensioned to prevent mechanical failure when the diode is subject to centrifugal forces.
AC generators used in aircraft have diodes to rectify the three-phase AC output of the exciter armature to DC for input into the main field. Generally, either three diodes or six diodes are mounted on the rotor shaft to provide the half wave or full wave rectification. The diodes can be positioned in several different arrangements, namely: an axial arrangement where the centerline of the diodes is parallel to and radially outward from the centerline of the rotor shaft; a perpendicular arrangement where the centerline of the diodes is at right angles to and radially outward of the centerline of the rotor shaft; and an angular arrangement where the centerline of the diodes is somewhere between parallel to and perpendicular to the centerline of the rotor shaft. The first-mentioned axial arrangement generally results in a smaller, lighter package and, therefore, is a desirable and frequently required mounting arrangement.
The great disadvantage of the axial arrangement is that a bending moment is applied to the stem of the diode resulting from the centrifugal force generated on the diode stem due to the mass of the stem itself, the lead wire and attaching mechanisms required to electrically connect the diode to either the exciter armature or main field. The diode stem must be electrically insulated from the base of the diode and this is commonly done by providing a disc of a glass-type composition between the base and the stem. The insulating member must have an expansion rate which closely approximates the expansion rate of the stem and base to enable the diode to perform throughout its temperature range without leaking or overstressing the insulating member.
Most of the aircraft generators in service today with diodes axially arranged have experienced mechanical failure from fracture of the insulating glass member which allows contaminants to enter the previously hermetically-sealed junction area resulting in diode shorting. In spray oil-cooled machines, the contaminant is the oil, itself. It is believed that this failure results from the bending moment applied to the diode stem resulting from centrifugal force and which reacts on the glass insulating member and causes the glass to fracture.